J-coupling between 17O and 1H in water (but not in O2) increases proton T2 relaxation rates. Thus, proton NMR techniques have the potential to detect and quantify oxidative metabolism by indirectly monitoring the conversion of 17O2 to H217O. 17O-decoupled proton NMR with a double-tuned solenoidal coil was used to quantify 17O in a semisolid tissue phantom consisting of 5% gelatin doped with 0.037, 0.2, 0.4, 0.8, or 2 atom % 17O-enriched water. The 17O decoupling RF pulse was applied during evolution of the proton echo in a standard spin-echo pulse sequence at 2T. The proton T2 relaxivity of 17O in the phantom was 3.3 sec-1 atom %-1, which is 84% of the theoretical maximum value; in contrast, the T1 relaxivity was <0.1 sec-1 atom %-1. Changes in proton signal intensity, which were assumed to reflect 17O decoupling efficiency, were determined for a range of decoupling amplitudes and echo times (TE). Decoupling efficiency reached a maximum value at a decoupling amplitude of 1-3 kHz (calibrated directly using 17O NMR). Decoupling efficiency, as determined by absolute increase in proton signal intensity, reached a maximum value at TEwln(1+R2T20f)/(R2f), where f is the 17O atomic fraction and T20 is proton T2 in the absence of 17O. However, if fractional increase in proton signal intensity was used instead of absolute increase, decoupling efficiency was a monotonically increasing function of TE. Propagation of error analysis was used to resolve this discrepancy. We found that TEwT20 minimized the uncertainty in the determination of the 17O concentration by the decoupling method. Using the optimized decoupling parameters, proton signal intensity increased by 12.4% in the tissue phantom containing natural abundance 17O (i.e., 0.037 atom %). The 17O decoupling method was incorporated into a spin-echo imaging pulse sequence and used successfully to quantify 17O concentrations in a cluster of isotope-enriched tissue phantoms. The optimized 17O-decoupled proton MR spectroscopy and imaging method is currently being applied to in vivo detection and quantification of H217O